An excerpt from the draft report teases the dangers of already hot subway platforms that are only projected to grow hotter:

Flooding is not the only climate change risk to the system. Extreme temperatures, particularly rising temperatures in the summer months, can stress the M.T.A. system. At higher temperatures, expansion joints on bridges and highways are stressed, and the instance of rail track stresses and track buckling increases. Underground, subway platforms and stations could become dangerously hot for riders.

Global warming is just part of the problem. Another major issue is the subway’s original design. Richard Barone, the director of transportation programs at the Regional Plan Association, tells Capital that New York City’s subway was built without adequate cooling and ventilation systems to begin with. (Holes were actually drilled into the sidewalks to cool it down.) And in the 110 years since, we’ve only saddled the infrastructure with more heat-producing sources, like trains fitted with more electronics and air conditioners.

Other major subway systems are experiencing similar heating problems. London has turned to lighter train cars, which generate less overall heat, and a host of cities around the world—including Dubai, Beijing, and Copenhagen—have deployed platform screen doors, which separate people on the platform from the hot train tracks. As for New York, the MTA is expected to release the full Transportation Reinvention Commission report soon, which should propose solutions to this white hot problem.

Air conditioning does generate extra heat actually. Their efficiency is not 100% so they don't just move heat. It's like your fridge, they also heat up the kitchen when cooling the inside of the fridge itself.